We estimated the monthly natural mortality rate (M) in the marine phase of Litopenaeus vannamei (Boone, 1931) and Farfantepenaeus californiensis (Holmes, 1900) in the Gulf of Tehuantepec, Mexico. The analyzed data were the commercial capture, the capture for unit effort (CPUE) and the fishing effort in trips for the brown shrimp (December 1989 to December 1998), and the white shrimp (October 1992 to December 1998). The models based on growth parameters were the most trustworthy for estimating the natural mortality rate (M). The results were: M= 0.20 monthly for the white shrimp and M= 0.22 monthly for the brown shrimp. The fishing mortality (F) increased from the 4 to the 16 months of age (F4= 0.02 F16= 0.79 monthly in white shrimp) and (F4= 0.18 F16= 1.61 monthly in brown shrimp). These results suggest a minor exploitation of the recruitment and greater on the reproducers groups of both species. In the fishing season 95/96, a decrease and an increase was observed in the captures of white shrimp and brown shrimp, respectively. This apparent tendency still stands according to that reported by El Instituto Nacional de la Pesca (INP) for the fishing season 03/04